1. FIELD OF THE INVENTION
The present invention relates to an apparatus and method for extruding and, more particularly, to an apparatus and method for extruding a shear thinning material.
2. STATE OF THE ART
In recent years, workers in the battery art have begun to understand and recognize the advantages of batteries manufactured from laminates including solid polymer electrolytes and sheet-like anodes and cathodes over conventional liquid electrolyte batteries. The advantages include lower battery weights than batteries that employ liquid electrolytes, longer service life, relatively high power densities, relatively high specific energies, and the elimination of the danger associated with batteries containing spillable liquid electrolytes such as acids. Until recently, commercial use of such batteries was limited by, among other things, the inability of such batteries to operate effectively except at relatively high temperatures.
More recently, however, laminate batteries using polymer electrolytes have been developed which possess good performance characteristics at or below room temperature. For example, in U.S. Pat. No. 4,925,751 to Shackle et al., a battery cell is described having good performance characteristics across a broad range of temperatures, including room temperature. The battery cell utilizes a current collector, such as a sheet of conductive metal foil, upon which is coated a thin layer, generally between 25 and 250 microns, of a cathode material.
The cathode material described, for example, in U.S. Pat. No. 4,925,751 to Shackle et al., is formed from a mixture of active cathodic material (usually a V.sub.3 O.sub.8 or a V.sub.6 O.sub.13 material), a conductive filler material (usually carbon particles), and an ionically conductive electrolyte material. In practice, such cathode material is paste-like in consistency and coating on the current collector is performed by conventional coating techniques such as a doctor blade or an extrusion method. The cathode material exhibits non-Newtonian fluid characteristics. Particularly, the cathode material is a shear thinning material, i.e., its viscosity decreases as the material is subjected to increasing shear stress. In still other words, the flow resistance of such materials decrease as shear increases. Apparatuses and methods for extruding a shear thinning cathode material must, therefore, be designed to accommodate this fluid characteristic.
In conventional mixing and extruding systems, a material is mixed to a homogeneous state and then is transported by a positive displacement pumping mechanism. The positive displacement pumping mechanisms typically transport the material under pressure through a narrow conduit to a reservoir having a larger cross-sectional area than the conduit. From the reservoir, the material is extruded through an extruder nozzle. In such systems, even if the rate of transport of the homogeneous material is somewhat irregular, the reservoir provides a consistent rate of extrusion.
Where, however, a shear thinning material is being mixed and extruded in such an apparatus, transport of the shear thinning material is substantially stopped when the material ceases to be under constant or increasing shear, i.e., when the homogeneous shear thinning material is expanded into the larger volume of the reservoir. In production of laminate batteries, it is necessary to achieve a constant thickness electrode laminate layer on a substrate to attain acceptable results in a battery and, consequently, a shear thinning electrode material must be extruded onto a substrate at a substantially constant rate. It is, therefore, desirable to provide an apparatus and method for mixing and extruding a shear thinning electrode material for a laminate battery in which electrode material is mixed to a homogeneous state and then transported while undergoing a constant or increasing shear to a point of extrusion such that it is extruded at a constant rate.